1. Field of the Invention
The present invention relates generally to light fixtures, and more particularly, to a light fixture useful in stage, television, motion picture, architectural lighting and the like, having an elliptical reflector and mechanical shutter to dim the light generated by an illumination source. This arrangement is an improvement over electrically controlled dimmers, enables the use of a discharge arc lamp as the illumination source, and provides for dimming the light produced by the illumination source over the range of from about 0 to 100% without causing any color change in the projected light.
2. Description of the Prior Art
Projection lighting for the media, entertainment and architectural industries is well known in the art and typically used for spot and soft-lighting applications. A common type is known as a fresnel based light unit, which has a basic design incorporating a large, planar filament lamp disposed at the center of a spherical reflector. A fresnel lens is axially displaced from the lamp/reflector combination to direct the light produced by the lamp from the apparatus. The distribution of light is altered by changing the distance between the lamp/reflector combination and the fresnel lens. In operation, the reflector collects light produced by the rear of the lamp and projects it forwardly to the lens. Light incident from the front of the lamp is directly transmitted to the lens, and the resultant cone of light produced is of a relatively wide angle, i.e., approximately 80-90 degrees. The light projected from the unit emanates in a generally spherical pattern. However, since the reflector only covers a relatively small portion of the spherical pattern, a significant amount of the generated available light does not enter the optical system and overall efficiency is reduced.
Typical prior art lighting units employ filament bulbs for the light source. These lamps produce relatively low luminescence, and operate at power levels of around 27 lumens per watt. Thus, a 600 watt lamp will produce a total lumen output of 16,200 lumens. A large filament lamp has a relatively low intensity, but reasonable longevity, on the order of 300 hours. If the filament is small, its intensity is higher. However, such lamps operate at high temperatures which results in a shorter life, typically on the order of around 75 hours. Such filament lamps are also disadvantageous in that they produce high levels of ambient heating. A Halogen bulb produces a very large proportion of heat relative to its total visible light output. Because of such operating conditions, they burn out and require frequent replacement. In addition, large filament lamps which are used in combination with spherical reflectors are not well suited to other reflector shapes, which can provide enhanced performance as described below. In view of this deficiency, it is desirable to use discharge arc lamps as the illumination source. Such lamps typically operate at around 90 lumens per watt, which results in much cooler operating temperatures thereby placing less demand on the air conditioning system for the room in which the lamp is placed. A 180 Watt arc lamp can produce a luminescence of 16,200 lumens. Because arc lamps are very small as compared to filament types, their intensities are higher. Nevertheless, arc lamps have very long operating lives, typically around 10,000-12,000 hours. However, discharge arc lamps may only be dimmed electrically to about 40% of maximum brilliance.
The design of these lighting units is driven by two primary considerations, the ability to dim the lamp over the full operating range of from about 0-100%, and the requirement to center the lamp about the optical center line of the unit. With filament lamps, dimming is facilitated by simply reducing the input voltage to the lamp. Although this is the most common method, it has a disadvantage in that reducing the voltage causes a temperature drop of the filament, which consequently changes the color of the light produced. Incandescent light tends to get redder as voltage is diminished, which is disadvantageous in the intended application of these units.
Lamp centering systems are embodied in various implementations. Some types are simply disposed on the lamp socket on a plate supported by a plurality of screws and springs. Adjustment of the screws is used to rock the lamp into an aligned position relative to the optical centerline. In other systems, an adjuster is employed to axially move the lamp in the reflector, and a clamp is used for vertical and horizontal translation. In general, however, most of these systems are cumbersome in operation and not very accurate.
In view of the state of the prior art, the present invention provides an improved lighting fixture for use in the above-described applications, which employs a specially designed reflector in combination with a mechanical dimming apparatus.